The present invention relates to an intrusion warning or alarm system for indicating the presence of an intruder within a given area. More particularly, the present invention relates to an improved intrusion warning system of the type wherein the presence of an intruder within a given area is detected by determining the changes caused by the intruder in an electric field in the area to be protected.
Various types of intrusion warning systems which operate on the principle of detecting a change in an electric field caused by the intruder are known in the art. Generally such systems utilize a high impedance sensing device, for example, an antenna, which is placed within the area to be protected and monitors the electric field within the area, which electric field may be either the inherently present electric field or an electric field specifically produced by the antenna or sensing device. Any change in the charge on the antenna due to the electric field being disturbed by the intruder is then detected and converted to an electrical signal which is used to provide an indication.
One type of intrusion warning or detection system which also responds to changes in electrical fields caused by an intruder and which operates in a different manner than that mentioned above, is disclosed in U.S. Pat. No. 3,237,105, issued Feb. 22, 1966 to Henry P. Kalmus, the subject matter of which is incorporated herein by reference. According to the teachings of this patent, a transmitting electrode and a receiving electrode are positioned on opposite sides of the area to be protected, e.g., a doorway, and a quasi-stationary electric field is produced within the area to be protected by connecting an oscillator to the transmitting electrode which oscillates at a frequency in the range of, for example, 5 to 100kHz. The receiving electrode is connected by an amplifier to an AM detector which detects the modulation of the received field signal caused by the movement of an intruder between the two electrodes, and the detected signal is fed to an amplifier including a bandpass filter with a bandpass in the order of 2 to 20Hz, thereby passing the low frequency component due to movement of an intruder in the area between the electrodes. This filtered signal is then fed to an indicating or alarm device to indicate the presence of an intruder.
Although the system disclosed in this patent operates satisfactorily in principle, the system is susceptible to a number of problems when attempts are made to utilize same in practical applications or to extend the range of the system so that it can be utilized to cover relatively large areas, thus necessitating that the sensitivity, and hence the gain, of the system be increased. One primary problem in practical applications of such systems is that of false alarms caused by stray electric fields in the area to be protected, or by transients. One primary source of stray electric fields normally present is the electric fields radiated by a.c. power lines or building wiring which would normally be present in the vicinity of an area to be protected by such an intrusion warning system. For example, while the primary a.c. power frequency, e.g., 60 Hz, would be outside of the passband of the system, the numerous devices and appliances normally operating from the power line cause extremely rapid current changes on the line, and these in turn cause energy to be created at multiples or harmonics of the power line frequency. If one of these harmonics should be equal to the frequency of the generated electric field or sufficiently close thereto to cause a beat frequency signal to be produced which is within the passband of the detection system, then a false alarm could result. Similarly, other transients creating signals within the passbands of the system could result in false alarms.